Radio Frequency (RF) receivers typically incorporate Automatic Gain Control (AGC) circuitry to provide proper conditioning of the received RF input power such that the received signal is kept within the usable dynamic range of the receiver. The information embedded in the received signal is transported using one of many different modulation schemes wherein the information may be contained in the frequency or phase of the received signal (e.g. FM, PM, FSK, PSK, etc.), in the amplitude of the received signal (eg. AM), or in both the amplitude and phase of the received signal (eg. QAM). Receiver AGC requirements are driven by several modulation and protocol parameters including, but not limited to, peak-to-average power ratio of the modulation, demodulator dynamic range limitations, analog gain/filtering compression (linearity) responses specific to the particular receiver, protocol driven slotted timing structures, and synchronous versus asynchronous system requirements. Examples of current protocol structures that incorporate distinct AGC system requirements are IS95 (commonly known as CDMA), GSM, iDEN, ETSI EN 300392 known as TErrestrial Trunked Radio (TETRA) and APCO 25. Another standard is the TIA 902 Scalable Advanced Modulation (SAM) standard that is a 700 MHz public domain standard that may be used for public safety applications. In present receivers, AGC system design is generally tailored to specific protocols and modulation strategies.
As has been previously noted, the modulation's peak-to-average ratio greatly influences the selection of the AGC threshold. If the information within the received signal is contained only in the phase component (e.g. FM or PM), the AGC strategy is greatly simplified since the modulated information is not lost even when the receiver is operating in compression. However, as increasing portions of the information are contained in the amplitude component of the received signal, as indicated by increasing peak-to-average ratios of the received signal, the receiver linearity requirements greatly increase thus necessitating increased AGC complexity. With increasing peak-to-average levels, the AGC thresholds are selected so as to keep the receiver operating completely out of compression. Compression is an operating state typically encountered in strong RF input power conditions where an amplifier stage looses its small signal gain characteristic. Compression results in the loss of all or a portion of the amplitude component of the modulated information. Therefore, as the peak-to-average ratio of the modulation increases, it becomes increasingly critical for the AGC to engage sufficient attenuation to prevent the amplifier from operating in a state of compression.
Another important aspect of the AGC system design involves trade-offs between attack time and tracking characteristics as determined by the modulation scheme of the received signal and by protocol specific timing requirements. The AGC tracking rate must be set to avoid distorting the received signal, particularly in the form of undesired amplitude ripple of the received RF carrier induced by the closed loop AGC continuously tracking the signal level. This distortion is particularly detrimental to a received signal containing significant amplitude component within its modulation. To reduce the AM distortion effect, the AGC tracking rate (which is inversely proportional to the closed loop bandwidth of the AGC) must be slowed down such that the AGC cannot respond quickly to amplitude variations in the RF carrier induced by the modulation scheme. Slow AGC tracking rates are desirable for highly linear modulation strategies that incorporate a large amplitude component within the RF carrier, since fast AGC tracking of linear modulation strategies will result in the AGC tracking out the desired amplitude portion of the modulation. However, in a simplified closed loop control system, slowing down the AGC tracking rate has the undesired effect of increasing the AGC attack time. The AGC attack time is the duration required for the AGC to engage the required attenuation to achieve proper demodulation once the receiver has encountered an arbitrary change in RF input power level. Most modern protocol structures require fast AGC attack times. For a basic closed loop feedback system, the fast AGC attack time requirement is in direct conflict with the requirement to minimize AGC induced amplitude distortion of the desired signal. Therefore, there exists a paradox in receiver systems, where particular protocols may require fast AGC attack times necessitating high AGC tracking rates, while the highly linear modulation strategy incorporated into the same protocol may require slow AGC tracking rates that would degrade attack times. Previous receivers have attempted to resolve this paradox by focusing on specific protocols and modulation strategies without regard to readily adapting the system to accommodate multiple protocols and modulation types. It is advantageous for receivers to be readily adaptable to accommodate the numerous modulation strategies and receiver protocols that exist today.
AGC strategies are further complicated by protocol requirements necessitating AGC response to both synchronous and asynchronous signals. Some of these modulation strategies have specific timing requirements where the desired data is contained within specific slots of time for a given duration. Such strategies are synchronous, and are known as Time Division Multiple Access (TDMA) protocols. The same protocol can define another mode which is asynchronous, allowing direct radio-to-radio operation. For example, the TETRA protocol defines a TDMA Trunked Mode of Operation (TMO) and a radio-to-radio mode of operation known as Direct Mode Operation (DMO). In the DMO mode, the receiver is required to receive a discontinuous TDMA signal from another radio. Thus, the receiver AGC settling time should be extremely fast. For multi-national wireless communication companies, it is a competitive advantage to define common platforms that are able to meet these diverse protocol timing and modulation linearity requirements. Therefore, the AGC operation must vary significantly for each of these standards, and the receiver hardware must be adapted for all targeted standards, protocols and modulation techniques. Existing systems that are targeted to multiple standards incur significantly increased costs and/or performance degradation due to increased hardware complexity and/or increased system resource demands (i.e. increased host processing resulting in increased power consumption and increased latency in servicing user specific applications).
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